This invention relates to arrangements, systems or receivers using monopulse techniques, such as those receivers used for radar surveillance or for radio frequency (RF) missile seekers, and more particularly to improved arrangements for locating targets, including up to two targets within the main beam of the antenna.
The problem of resolving more than one scattering center in a radar beam draws interest from many radar applications such as tracking, target recognition, and surveillance. When two scattering centers fall within the same range-doppler cell, their complex amplitude interfere with each other, causing a phenomena called xe2x80x9cglint,xe2x80x9d where the indicated angle of the target wanders wildly. One technique to alleviate this problem is to resolve the scattering centers in range using a wide bandwidth waveform (HRR). However wideband processing is expensive and there are technological limits to this technique. Furthermore, in an electronic counter measure (ECM) scenario where there is a jammer whose radiation is present in each range cell, range resolving techniques would not help. Therefore it is desirable to develop techniques to resolve two sources, reflective or radiating, in angular dimensions.
There have been extensive studies in this direction. Monopulse processing techniques for multiple targets are discussed in xe2x80x9cMultiple Target Monopulse Processing Techniques,xe2x80x9d by Peebles and Berkowitz, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-4, No. 6, November 1968. However, the technique disclosed therein requires special antenna configurations that are much more complicated than the sum-difference channels normally used in monopulse radars. Moreover, the proposed technique generally requires six beams to resolve two targets. The article xe2x80x9cComplex Indicated Angles Applied to Unresolved Radar Targets and Multipath,xe2x80x9d by Sherman, IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-7, No. 1, January 1971, concludes that with a conventional monopulse configuration, a single pulse solution is impossible. This reference discloses a method to resolve two targets using two independent measurements, but it is not a xe2x80x9cmonopulsexe2x80x9d technique per se.
Techniques have been developed based on the PRIME-MUSIC and ESPRIT algorithms, but again these techniques require multiple measurements. Such techniques are disclosed in xe2x80x9cA Class of Polynomial Rooting Algorithms for Joint Azimuth/Elevation Estimation Using Multidimensional Arrays,xe2x80x9d by G. F. Hatke and K. W. Forsythe, 28th Asilomer Conference on Signals, Systems and Computers, Pacific Grove, Calif., 1994; and xe2x80x9cESPRIRTxe2x80x94Estimation of Signal Parameters Via Rotational Invariant Techniques,xe2x80x9d by R. Roy and T. Kailath, IEEE Transactions on Acoustics, Speech, Signal Processing, Vol. 37, pp. 984-995, July 1989. Techniques using multiple pulses may suffer from the target fluctuations between pulses. Also, there may not be time for multiple measurements, especially when pulse compression is used to generate fine rangexe2x80x94doppler profile.
An object of this invention is to provide a method and apparatus for resolving two sources in the same range-doppler cell in a monopulse radar beam.
Another object of the present invention is to use the conventional monopulse radar antenna configuration and literally a single pulse radar measurement to achieve two target resolution.
These and other objective are attained with a method and system for identifying the locations of plural targets lying within the main beam of a monopulse antenna including four ports for generating sum, elevation difference, azimuth difference, and double difference signals. The method comprises the steps of processing the sum, elevation difference, azimuth difference, and double difference signals in accordance with a series of linear equations to obtain a set of intermediate values; and processing those intermediate values in accordance with a set of algebraic equations to obtain signals representing an angular direction of each of the plural targets.
Preferably, the method further includes the step of processing the sum, elevation difference, azimuth difference and double difference signals and the signals representing the angular directions of the targets according to a further set of algebraic equations to obtain signals representing the amplitude of the beam reflected from each of the targets. Also, in a preferred embodiment, the signals representing the angular direction of the targets include signals representing, for each of the targets, an angular direction of the target in an x-plane, and an angular direction of the target in a y-plane.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.